hazy nights, and even daylight, an estimate can be formed of the laws of distribution of the stars of various orders, proceeding onwards from those included in my chart of 324,000 stars, and thus extending and amplifying the teaching commenced in that chart. When that work has been accomplished, we shall begin to understand the real wonders of the star-depths, the magnificence of the subordinate starschemes which have been mistaken for the sidereal system itself, and something of the grandeur of that system, whose limits lie far beyond the range of our most powerful telescopes, while within them are included all the various orders of celestial objects which the telescope has yet revealed. Combining such lessons with what has been learned and yet remains to be learned of the movements taking place within the sidereal universe, we shall have a picture grander and more impressive by far than any yet presented to our contemplation; we shall learn that the true Galaxy is infinitely more extended, infinitely more complex in structure, than we have supposed, and that the processes at work within its bounds are infinitely more stupendous.

307

TRANSITS OF VENUS.

DURING the autumn months of 1882 the evening star was seen drawing nearer and nearer, night after night, to the place of the sun, until at length she set too soon after him to be discernible save with the telescope. To the astronomer this approach of Venus to the sun had an interest greater than usually attaches to the phenomenon; for it was known that she would not pass from the eastern to the western side of the sun's orb without crossing his face. The passage of Venus close by the sun is, of course, a phenomenon of frequent occurrence and possessing no special interest. Hesperus, the star of evening, cannot change into the morning star, Lucifer, without passing the sun's place upon the heavens; nor can Lucifer change into Hesperus without a similar passage; though there is a distinction between the two cases, for it is by passing between the sun and the earth that the evening star changes into the morning star, while it is by passing beyond the sun, so that the sun comes between the earth and her, that Venus changes from a morning star into an evening star. But these are astronomical phenomena which have been witnessed and understood for thousands of years. It is when Venus, in passing from the east to the west of the sun, does not steer clear of his disc, but traverses it, so that she appears in the telescope like a round black spot upon his face, that every astronomer is interested. For these occasions are few and far between. The last transit of Venus before 1874 occurred more than 105 years earlier; and although that transit was followed by

another in 1882, yet after that second transit an even longer interval will elapse before another occurs, than has passed since the transit of 1769: not until June 2004 will Venus again pass over the face of the sun. And besides the interest naturally attaching to a phenomenon which occurs soseldom, the transits of Venus have a scientific importance depending on their relation to celestial measurement, since they afford the best means astronomy possesses for determining the distance of the sun, and with that distance the dimensions of the solar system, besides whatever information we may hope to possess respecting the tremendous distances which separate the sun from his fellow-suns, the stars.

Transits of the inferior planets were, at first, only awaited with interest because of their bearing on the Copernican theory of the solar system. It was not until astronomers had abandoned the old systems that they could have any positive assurance that any planet ever passed between the earth and the sun. Moreover, regarding the celestial bodies as all self-luminous, astronomers could hardly have expected that, even if a planet passed across the sun's face, it would be discernible as a dark spot. We do, indeed, hear that some old observations of sun-spots were regarded as transits of Mercury or Venus across the sun's disc. Thus, the author of the Life of Charlemagne' tells us that Mercury was seen in April 807, as a black spot upon the sun's face, for eight consecutive days. Kepler, who was perfectly well aware that Mercury could not remain as many hours on the sun's disc, endeavoured to show that the expression used in the manuscript of the old writer might not have been octo dies, but octoties, a barbaric form of octies for eight times. Again, the famous physician Ebn Roschd (commonly called Averroës) says, in his Ptolemaic Paraphrase, that in the year 1161 he saw Mercury on the sun at a time when the planet really was in inferior conjunction (that is, passing between the earth and the sun). Kepler himself believed that he had so seen the planet. In his day Mercury was supposed to be a much larger body than it actually is. Hence there was

nothing surprising in the fact that an experienced astronomer like Kepler should have mistaken for the planet a sunspot, seen no doubt only for a short time when the sun was low down. But later, when the telescope had revealed the existence of spots upon the sun's face, Kepler admitted that in all probability he had seen such a spot, and not Mercury. We know now that even Venus, much larger though she is than Mercury, and much nearer to the earth when in transit, is quite invisible to the unaided eye at such a time.

Gassendi, who was the first to witness a transit of an inferior planet, saw Mercury pass across the face of the sun on November 7, 1631. His account of the observation is quoted somewhat fully in the Cornhill Magazine for November 1868. Kepler, who had announced the transit, had also predicted a transit of Venus on December 6, 1631; and Gassendi hoped to witness this event. Kepler predicted that the transit would begin shortly before sunset; but as the transit of Mercury had not occurred exactly at the time indicated by Kepler, Gassendi thought that quite possibly he might witness the whole of the transit of Venus. He was prevented from observing the sun on December 4 and 5 by impetuous storms of wind and rain. 'On the 6th he continued to obtain occasional glimpses of the sun, till a little past three o'clock in the afternoon, but no indication of the planet could be discerned.' 'On the 7th he saw the sun during the whole forenoon, but looked in vain for any trace of the planet.' We now know that the transit took place during the night between December 6 and December 7.1

It is commonly stated that no part of the transit could have been witnessed in Europe. The present writer, however, having calculated the circumstances of the transit as accurately as the case warrants, finds that the end of the transit could have been seen from the south-eastern parts of Europe, occurring at sunrise for all places on a line drawn from Gibraltar through Marseilles, Dresden, St. Petersburg, to the extreme north-east of European Russia. The account of Gassendi's failure in the excellent treatise Les Passages de Vénus, by M. Dubois, Naval Examiner in Hydrography for France, is amusing: 'Le passage de Vénus,' he says, 'qui sans doute n'était pas prédit avec une pré

Just as the transit of December 1874 was followed by another December transit in the year 1882, so the transit. of December 1631 was followed by another in December 1639. The earlier had escaped observation, as we have seen, though predicted by Kepler; the latter, which according to Kepler's tables would not take place, was observed, though it almost escaped the ingenious astronomer who detected the mistakes in Kepler's computations and watched for its occurrence. This astronomer was one whose name is only not associated with any great discoveries because he died so young. Had he lived it is probable that Newton himself would not have stood much higher among the astronomers of England. Jeremiah Horrocks, minister of Hoole, in Lancashire (aged only twenty), had in his zeal for science gone over the computations published by Kepler in the Rudolphine Tables. Comparing these with Lansberg's Tables of the Motions of Venus, he noticed that, while according to Kepler the planet would pass very close to the sun, but south of his disc, on December 4, 1639, Lansberg's Tables assigned to the planet at that conjunction a course traversing the northern part of the sun's disc. He had found Kepler a much more reliable authority than Lansberg; but he had reason, from his own observations, to believe that Venus would follow a course between the two paths thus assigned by Lansberg and Kepler-somewhat. nearer to Kepler's—insomuch that, instead of passing south of the sun, she would transit the southern part of his disc. He determined, therefore, to watch carefully for this interesting phenomenon. 'Lest a vain exultation should deceive me,' he says, ' and to prevent the chance of disappointment, I not only determined diligently to watch the cision suffisante, ne fut pas observé, d'abord parce que Gassendi, qui s'apprêtait à l'observation, en fut empêché par la pluie, mais surtout parce que le passage eut lieu pendant la nuit pour les observateurs Européens.' It may reasonably be admitted that the occurrence of the transit when the sun was below the horizon was a sufficient cause for Gassendi's failure, apart from the rain.